As is known, an automotive water pump comprises a pump body; a drive shaft; and an impeller housed inside the pump body and fitted to the shaft. The shaft is operated by a pulley, and is fitted through an opening in the pump body, in which it is supported on a bearing.
Sealing between the shaft and the pump body is normally performed by a mechanical seal fitted to the part of the shaft extending axially between the impeller and the shaft bearing.
The mechanical seal comprises a first sealing ring fitted integrally to and rotating with the shaft; and a non-rotating second sealing ring connected to the pump body. The two sealing rings cooperate axially with each other to form a sliding face seal.
More specifically, the second sealing ring is secured to a metal cup, designed to fit inside the opening in the pump body, by means of a bellows of elastomeric material fixed at one end to the cup and at the other end to the second sealing ring; and the first sealing ring is fitted to a sleeve, fitted to the shaft, with the interposition of a ring of elastomeric material.
The mechanical seal also comprises a spring housed inside the cup and compressed between an end wall of the cup and the second sealing ring to push the second sealing ring, with a predetermined work load, against the first sealing ring.
For the seal to function properly, a fluid gap must be ensured where the two sealing rings contact, to prevent severe wear, overheating, and noise. One method of producing ceramic, e.g. silicon carbide, sealing rings with improved surface characteristics is described in WO2007/057934.
Though normally excellent in terms of noise, sealing rings produced using the above process are subject, in the event of poor lubrication, to stick-slip phenomena, which are aggravated by the elasticity of the bellows of elastomeric material, and may result in self-induced torsional oscillation and, therefore, noise.
Demand therefore exists within the industry for further improvement to eliminate noise at the source.